Streamer Development Research Articles

Discharge Characteristics and Numerical Simulation of the Oil–Gas Surface under DC Voltage

Low insulation strength at the oil–gas surface due to oil leakage and partial discharge of oil-immersed power equipment is a major threat to the safe and reliable operation of power systems. This paper investigates the initiation and development of the oil–gas surface discharge. The oil–gas surface discharge test platform was established, and discharge tests were carried out at different gap distances (1–2.5 mm). By coupling the electric field and flow field, the multi-layer dielectric discharge streamer model was built, and the characteristics of charge and electric field distribution at different gap distances were studied. The test results show that the liquid surface between the electrodes rises during the discharge process. Furthermore, the surface discharge voltage exceeds the air gap discharge voltage. With the simulation analysis, the oil–gas surface discharge is a typical streamer development process. Under 50 kV applied voltage and 2.5 mm gap distance, the average development speed of the streamer is 12.5 km/s. The larger the gap distance is, the greater the average streamer development speed is. The recording and numerical simulation of the discharge process are of great significance for exploring the mechanism of oil–gas surface discharge, optimizing the discharge process, and diagnosing partial discharges.

Insight into the dynamic evolution behavior of subsonic streamers in water and their voltage polarity effect

Due to the complex interaction between liquid, gas, and plasma, the pre-breakdown process in water under quasi-static moderate electric fields, namely the development of subsonic streamers, was unclearly understood so far. In this paper, the dynamic evolution behavior of subsonic streamers and their voltage polarity effects were investigated. It was indicated that the whole streamer development process can be divided into two successive stages: bottom-up period characterized by root spherical expansion and OH (309 nm) emission line; top-down period characterized by head burst expansion and Hβ (486 nm), Hα (656 nm), and O (777 nm) emission lines. Further analysis revealed that the magnetic pinch effect on the internal plasma distribution determines the expansion mode of the streamer. The low capture energy of the solvated electron and local space charge accumulation make the positive streamer propagate faster at a low voltage level. However, the limited carrier resource and relatively divergent internal plasma distribution (weak magnetic pinch effect) hinder the propagation acceleration of the positive streamer with the applied voltage. Thus, the voltage polarity effect variation can be observed at high voltage levels. Finally, a novel framework model was proposed to depict the dynamic evolution behavior of subsonic streamers. Our results can provide a deeper insight into the electrohydrodynamics of dielectric fluid and promote the relevant industry applications.

Numerical Simulation of Streamer Development Process in N2/H2 Environment at Elevated Temperature

In this article, a two-dimensional (2-D) axisymmetric fluid model is built to investigate the streamer development characteristics, as well as the distribution time of each reactive components with pure N <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{2}$</tex-math> </inline-formula> , pure H <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{2}$</tex-math> </inline-formula> , and mixed gases of 50% N <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{2}$</tex-math> </inline-formula> and 50% H <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{2}$</tex-math> </inline-formula> at lower voltage and elevated temperature. The results indicate that in terms of thermal breakdown, the insulating properties of pure N <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{2}$</tex-math> </inline-formula> are stronger than that of pure H <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{2}$</tex-math> </inline-formula> . Under the influence of high temperature, the shape of the ionization rate distribution is no longer a slender wave shape but more spread. In addition, a new ionization area will be formed near the rod electrode, and finally, the ionization region near the rod electrode will merge with the ionization wave head with the increase in temperature. For the mixed gas of 50% N <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{2}$</tex-math> </inline-formula> and 50% H <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{2}$</tex-math> </inline-formula> , it will dissociate into atoms and recombine under the action of high temperature to form molecules containing both nitrogen and hydrogen elements, such as NH <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{3}$</tex-math> </inline-formula> . NH <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{3}$</tex-math> </inline-formula> is an electronegative gas, which will greatly enhance the thermal breakdown performance of the mixed gas and makes its dielectric strength second only to pure N <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{2}$</tex-math> </inline-formula> . Therefore, it can be speculated that appropriately increasing the proportion of H <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{2}$</tex-math> </inline-formula> in the mixed gas will improve both the arc extinguishing ability and the postarc thermal breakdown performance, which has been proven by experiments considering that the arc extinguishing ability of H <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{2}$</tex-math> </inline-formula> is stronger than that of N <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{2}$</tex-math> </inline-formula> .

Multiple Auroral Streamer Bundles in Conjugate Hemispheres and Multiple Near‐Earth Injections

AbstractThe auroral streamer is a type of auroral form commonly observed during geomagnetic substorms. Previous studies suggested a coupling between auroral streamers and channels of bursty bulk flows in the plasma sheet. However, whether one flow channel can map to multiple streamers is unclear. Here, we present an event containing consecutive auroral streamer subevents. The event features similar spatial and temporal development of auroral streamers in conjugate hemispheres. In most of the subevents, we observed that what an in‐space auroral camera saw as one streamer actually consisted of multiple streamers (i.e., a streamer bundle). A coordinated analysis of near‐earth injections and ionospheric currents suggests that one overall flow channel (or bubble) can map to a streamer bundle. Multiple streamer bundles and thus multiple overall flow channels can occur simultaneously at different local times. Evidence supports that the overall flow channel may consist of or split into several narrower flow channels and each maps to a streamer and causes a particle injection.

Positive and negative streamers in air and nitrogen in a sharply inhomogeneous electric field under conditions of runaway electron generation

AbstractThe development of positive and negative streamers in a point‐to‐plane gap filled with air and nitrogen at various pressures (50–200 kPa) and voltages (8–25 kV) was studied. A four‐channel intensified charge‐coupled device (ICCD) and a streak‐camera were used. Electrical parameters were measured with high resolution (10 GHz). An original method of measuring a displacement current caused by a streamer was applied. As was expected, positive streamer branches in nitrogen at low voltages and/or elevated pressures while a large‐diameter streamer is formed in air. However, at high voltages or negative polarity, the large‐diameter streamer is formed both in nitrogen and air in the entire pressure range. It was found that runaway electrons (REs) are generated in the very first picoseconds of gas ionisation near the pointed cathode. It was assumed that REs can be generated near the pointed anode and produce bremsstrahlung radiation due to a sharply inhomogeneous distribution of electrical potential. It was found that in the final stage of negative streamer development in air and nitrogen, the gas between the streamer front and the opposite electrode is ionised almost simultaneously in the entire volume when the streamer diameter is almost equal to the interelectrode gap.

Studying the Features of Streamer Formation and Development in Air by Using Computation Experiments

The aim of the work is to estimate the relevance and current state of the problem of mathematical modeling of streamer discharges in air, as well as to identify the features pertinent to formation and initial development stage of two-head streamers in air by using computational experiments. The streamer discharge mathematical model developed at the National Research University Moscow Power Engineering Institute Department of High Voltage Engineering and Electrophysics was used as the research tool. It is shown that mathematical modeling of a streamer discharge can be effectively used in its studies and in training specialists in the field of electric power engineering and high-voltage electrical technologies. The article presents the results of computational experiments on studying the occurrence of streamers in a discharge gap filled with dry air with a uniform electric field (EF) under standard atmospheric conditions. The initial conditions were chosen so that a single-avalanche-streamer formation mechanism was realized. It is shown that in the considered range of initial conditions, the distance between the anode and initial inhomogeneity has a weak, within 1%, influence on the minimum and maximum values of electric field strength in the streamer. A different picture is observed on the anode surface. When a two-head streamer is formed in the discharge gap depth, the field strength at the anode remains close to the average strength in it. As the negative streamer head approaches the anode, the field strength on its surface increases to values close to those achieved in the positive head. After that, the field at the anode weakens rapidly to a level at which effective impact ionization becomes impossible. The article will be of interest for specialists and graduate students working in the field of electric power engineering and high-voltage electrical technologies.

The flashover of epoxy initiated by micron metal particles under DC voltage: phenomenon and mechanism

Epoxy post-insulator is one of the key parts in SF6 gas-insulated DC wall bushing, which is irreplaceable in high voltage direct current transmission projects. Flashovers occur on post-insulators frequently, where a great number of tiny metal particles exist. The micron metal particles attached to the epoxy will change the insulation state of the surface. However, this relation between microstructure of material and macroscopic electrical properties on flashover would still arouse controversy. In order to study the effect of particles on the flashover characteristics, the particles generated from wear of spring in DC wall bushing were selected, the surface potential decay along surface and the DC flashover voltage of epoxy attached with particles were measured. The results show that the discrete particles could increase surface trap level by 0.025 eV under the effect of Van der Waals force. Furthermore, the deeper traps could capture the charge during the streamer development and inhibit the flashover, when the particle amount less than 500 per mm2. If the particles are more enough to form the continuous paths, these conductive paths could promote the streamer to propagate, which shortens the insulation distance, increases the electric field, and decreases the flashover voltage by 50% finally.

Evolution of positive streamers in air over non-planar dielectrics: experiments and simulations

We study positive streamers in air propagating along polycarbonate dielectric plates with and without small-scale surface profiles. The streamer development was documented using light-sensitive high-speed cameras and a photo-multiplier tube, and the experimental results were compared with 2D fluid streamer simulations. Two profiles were tested, one with 0.5 mm deep semi-circular corrugations and one with 0.5 mm deep rectangular corrugations. A non-profiled surface was used as a reference. Both experiments and simulations show that the surface profiles lead to significantly slower surface streamers, and also reduce their length. The rectangular-cut profile obstructs the surface streamer more than the semi-circular profile. We find quantitative agreement between simulations and experiments. For the surface with rectangular grooves, the simulations also reveal a complex propagation mechanism where new positive streamers re-ignite inside the surface profile corrugations. The results are of importance for technological applications involving streamers and solid dielectrics.

The influence of humidity on positive streamer propagation in long air gap

A 2D numerical simulation of the positive streamer properties was performed in 9–12 cm plane-to-plane air gaps for various pressures and water vapor contents. It was shown that an increase in air humidity leads to hampering the streamer development and to increasing the average critical electric field required for bridging the discharge gap. The effect of humidity was most profound at atmospheric pressure and decreased with decreasing pressure. The influence of water content on the streamer properties was explained by a decrease in the streamer channel conductivity due to dissociative recombination of electrons with positive hydrated ions and enhanced three-body electron attachment to O2 molecules. The calculated critical electric field and streamer velocity in humid air gaps were compared with available experimental data.

Numerical Analysis of Streamer Discharge Propagation Characteristics in Natural Ester

Natural ester is viewed as the most promising future alternative to mineral oil due to its high degradability, but its inferior breakdown characteristics prevent its application of large power equipment. The purpose of this research is to investigate the breakdown characteristics of natural ester. First, the natural ester's ionization potential and molecular number density are calculated. Then, an improved natural ester streamer discharge model is developed using the Poisson's equation and the carrier continuity equation, taking into account the effect of temperature on carrier mobility and incorporating the computed ionization potential and other factors. The results for different lightning impulse voltages were analyzed by COMSOL software. Comparing the mineral oil, the results indicate that the natural ester breakdown voltage is close to and slightly lower than that of the mineral oil but the acceleration voltage is very similar to the breakdown voltage. The lower ability of esters in prevention against the development of fast and energetic streamers is consistent with the description of the experimental results. In addition, the model takes into account the effect of temperature on the carrier mobility. As the applied voltage increases, the change of temperature in the streamer channel accelerates the carrier motion and promotes the propagation of the streamer. © 2022 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Discharge Mechanism Difference Analysis Between Natural Ester and Mineral Oil Under Impulse Electric Field: A DFT Investigation

The lightning impulse breakdown strength of natural ester is lower than that of mineral oil. This article focuses on a discharge mechanism difference analysis of natural ester and mineral oil under a strong impulse electric field (EF). Based on density functional theory (DFT), the ionization energy, electron affinity (EA), and excitation energy of natural ester and mineral oil were calculated under different EF strengths. The microparameters related to the discharge of natural ester and mineral oil were compared, showing that the former has a lower ionization energy and a larger dipole moment. This quality makes natural ester more likely to ionize and produce electrons, which more easily promotes the formation of electron avalanches and streamer. Meanwhile, natural ester molecules are more prone to transition from the ground state to the excited state than mineral oil molecules, exacerbating the photon release and formation of discharges in natural ester. The greater conductivity of natural ester leads to its dispersion and the rapid development of discharge streamers. In addition, an electrostatic potential (ESP) analysis on molecular surfaces was used to predict the active sites of the oil molecular structure. This article provides help for understanding the relationship between the micromolecular structure and the macroscopic discharge characteristics for different types of oil molecules.

Experimental investigation on streamer inception from artificial hydrometeors

In this study we use an experimental investigation to shed light on the lightning inception problem. From atmospheric observations, it is known that the electric fields in thunderclouds are significantly lower than required for electric breakdown in air. One theory to explain lightning inception is that hydrometeors, i.e. any liquid or solid water particles formed in the atmosphere, greatly enhance the local electric field and can thereby initiate an electron avalanche leading to a streamer discharge. In this study, we investigate streamer initiation in the presence of artificial particles with different shapes. A metal or dielectric (TiO2) particle is suspended between a high-voltage and a grounded planar electrode which are separated by 16 cm in 50 mbar air. The particles are shaped as ellipsoids with a length of 8, 4, 2, and 1 cm and with different aspect ratios. A negative high voltage pulse is applied with a rise time of 30 ns, a pulse width of 1–10 μs, a repetition rate of 1 Hz, and a maximum voltage between 1 and 50 kV. Results show that the required background electric field for breakdown in the presence of a dielectric particle is decreased to 0.4 times the air breakdown field. Moreover, we observed bipolar streamer development from the particles where negative streamers are thicker and slightly slower than positive streamers. Finally, we found that streamers from longer particles are thicker and faster.

Al2O3-Epoxy Flashover Characteristics Under Positive and Negative Nanosecond Impulse in SF6

Surface flashover of dielectric materials often causes extensive damage to pulse power devices. Nowadays, this insulation issue is exacerbated by the demands for ever-higher operating voltages. To solve this issue, the flashover voltage and the streamer development characteristics were investigated under positive and negative nanosecond impulse, respectively, in pressured SF6 gas. First, the surface flashover strength was obtained and the Weibull distribution is introduced to describe the statistical characteristics. The results showed that the negative flashover voltage was much higher than the positive one for Al2O3-epoxy. Within the range of 0.1–0.3 MPa, the negative voltage increased with the gas pressure, while the positive one performed independent with pressure. Then, to investigate the streamer evolving process, a high-speed camera was used to get the discharge picture and the instant velocity of streamers at different conditions. Surface morphology change after the flashover was also analyzed. At last, the possible mechanisms for polarity and pressure effects were given.

Charging Mechanisms and Models for Nanoparticles Suspended in Liquid Dielectrics

Liquid dielectrics filled with nanoparticles have drawn huge attentions due to enhanced dielectric and thermal properties. Despite of great achievements have been made, the charging mechanisms of nanoparticles suspended in liquid dielectrics are still open questions. It is previously argued that field charging with Maxwell–Wagner relaxation fails to explain the enhanced breakdown strength when nanoparticles have longer relaxation time constant than the timescale of streamer development in liquid dielectrics. However, it is proven in this article when the condition that the charging time constant <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\tau _{\mathrm {pc}} \ll $ </tex-math></inline-formula> the charging time <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$t_{\mathrm {pc}}$ </tex-math></inline-formula> is fulfilled, nanoparticles can acquire significant number of charges even if <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$t_{\mathrm {pc}} \ll $ </tex-math></inline-formula> relaxation time constant <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\tau _{\mathrm {MW}}$ </tex-math></inline-formula> . It is also showed that the field charging models for nanoparticles suspended in gases and liquids share the same formulation and thus can be unified. In addition, diffusion charging is proposed to be one of the charging mechanisms for nanoparticles in liquid dielectrics, and Fuchs’s model is extended from aerosol science to liquid dielectrics. It is demonstrated the diffusion charging could be far more important than field charging in liquid dielectrics. This work may bridge the gaps of charging mechanisms and models between gaseous and liquid dielectrics.

Dynamic development model for long gap discharge streamer-leader system based on fractal theory

Establishing a long air gap discharge model considering the streamer-leader transition and randomness of the discharge path is of great significance to improve the accuracy of discharge characteristic prediction and optimize external insulation design. Based on fractal theory and thermal ionization theory of streamer-leader transition, this work establishes a dynamic development model for the long air gap discharge streamer-leader system, which includes streamer inception, streamer development, leader inception, development of streamer-leader system and final jump. The positive discharge process of a 3 m rod plate is simulated to obtain the fractal distribution of the discharge path and the law of leader development for comparison with the discharge test results. The results show that the simulation model is similar to test results in the development characteristics of leader path distribution, each stage time and leader velocity. Finally, a simulation calculation of a 50% breakdown voltage of the rod-plate gap and ball-plate gap is carried out, with results fairly consistent with test data, proving the effectiveness and practicality of the model.

Streamer Propagation along Profiled Insulator Surfaces under Positive Impulse Voltages

Controlling discharge growth on insulator surfaces is important in high voltage gaseous insulation systems. In this study, the effect of small-scale surface profiles on streamer discharge propagation is examined experimentally. The experimental test objects were 5x72x150 mm polycarbonate plates with and without machined surface profiles. One test object had a surface with 0.5 mm deep semi-circular corrugations, while the other profile had 0.5 mm deep rectangular corrugations. The semi-circular profile increased the surface area with 20 %, while the rectangular profile increased the area with 110 %. A plain surface was also examined as a reference. Positive impulse voltages were applied to a 1 mm thick disk electrode placed 2 mm above the insulator. The insulator was placed in a grounded aluminium casing. The streamer development was imaged with a light-sensitive high-speed camera. Surface charges left on the surface after the impulse were examined using an electrostatic probe and simulations of saturation charge. The rectangular surface profile reduced the streamer range significantly, which suggests an effect of added surface area. Imaging indicated that the wavelike surface streamers follow the profiles closely. Surface potential measurements showed a saddle-shaped distributions, with values in line with saturation charge computations.

Negative Streamer Propagation in Nitrogen

For the development of negative streamer, a one dimension simulation is presented when a negative electric field is applied at atmospheric pressure to a 4 mm gap in nitrogen. At applied electric fields of 55, 60, 65 and 70 kV/cm, streamer parameters were studied at various time intervals. The aim of this paper is to determine the minimum electric field that must be applied for stable propagation of negative streamer discharge in nitrogen gas. As functions of position and time, the calculations provide detailed electron and ion density predictions, electric fields and density of space charges. The time interval was with a nanosecond resolution. Using 8000 element mesh to resolve the characteristics of the streamer, spatial resolution over the separation of electrodes 4 mm was obtained. The results were obtained by solving the equation of continuity for electrons and ions and the Poisson equation solution.

Dynamic evolution of liquid phase disturbance and its critical influence on pre-breakdown process

Liquid phase disturbances are often observed in pre-breakdown processes; however, their dynamic behaviors are rarely studied. In this paper, time evolution characteristics of liquid phase disturbance under ultra-long pulses (&amp;gt;100 ms) were investigated. The results showed that the steady expansion of liquid phase disturbance follows the pattern of constant heating power, volume growth rate, and liquid temperature (about 52 °C unvaried with applied voltage). The shrinkage of liquid phase disturbance with the applied voltage leads to the breakdown transition from a full disturbed phase mode to a partial disturbed phase mode. Further research indicated that the liquid phase disturbance has a significant influence on the development of subsonic streamers (especially for positive polarity). In the disturbed phase of liquid, the streamers propagate faster with a plump morphology than in the stationary phase. The local turbulences at the boundary of the disturbed phase can retard the streamer propagation remarkably and lead to the streamer branching. Finally, the abnormal downtrend of positive streamers' average velocity varied with the applied voltage due to the shrinkage of liquid phase disturbance was predicted and observed for the first time.

Streamer formation processes trigger intense x-ray and high-frequency radio emissions in a high-voltage discharge.

For a laboratory discharge initiated in a long air gap by a microsecond megavolt pulse, we simultaneously register wideband high-frequency microwave and hard-x-ray emissions and thoroughly analyze the temporal relationship of the emissions depending on the discharge evolution. The temporal structure of microwave radiation is found to consist of numerous short intense bursts with high-frequency components. We directly show that x-ray and microwave emissions can appear almost synchronously in the discharge but only when a complex net of countless plasma channels forms and spans the entire discharge gap. The channel formation is closely related to the intense development of multiple streamers.

3D particle simulations of positive air–methane streamers for combustion

Streamer discharges can be used as a primary source of reactive species for plasma-assisted combustion. In this research we investigate positive streamers in a stoichiometric air–methane mixture at 1 bar and 300 K with a three-dimensional particle-in-cell model for the electrons. We first discuss suitable electron scattering cross sections and an extension of the photoionization mechanism to air–methane mixtures. We discuss that the addition of 9.5% methane leaves electron transport and reaction coefficients essentially unchanged, but it largely suppresses photoionization and shortens the photon mean free path. This leads to (1) accelerated streamer branching, (2) higher electric field enhancement at the streamer head, (3) lower internal electric fields, and (4) higher electron densities in the streamer channel. We also calculate the time-integrated energy density deposited during the evolution of positive streamers in background electric fields of 12.5 and 20 kV cm−1. We find typical values of the deposited energy density in the range of 0.5–2.5 kJ m−3 within the ionized interior of streamers with a length of 5 mm; this value is rather independent of the electric fields applied here. Finally we find that the energy deposited in the inelastic electron scattering processes mainly produces reactive nitrogen species: N2 triplet states and N, but also O and H radicals. The production of H2 and O2 singlet states also occurs albeit less pronounced. Our calculation of the primary production of reactive species can for example be used in global chemistry models.